One type of sensor known in the prior art is an anemometer, which is useful for measuring the fluid flow velocity of the medium. For example, in hot wire anemometry, a filament is exposed to the medium. A current is developed through the filament to heat the filament to a preselected temperature. As the medium flows past the filament, it has a cooling effect thereby dissipating heat from the filament. The amount of heat dissipated is a function of the fluid flow velocity. If the temperature of the filament is kept constant, it is obvious to one skilled in the art that the amount of current required to maintain the constant temperature of the filament would then be a function of the fluid flow velocity of the medium.
One such prior known integrated circuit anemometer utilizes the temperature sensitivity of integrated resistors. The integrated resistors are configured in a Wheatstone bridge circuit. When an air flow is present across the sensor, the resistors, which are heated by the bridge current, will be cooled. The temperature decrease of the resistors which are normal to the flow is somewhat larger than the temperature decrease of the resistors which are parallel to the flow. Because the value of the diffused resistors are a funtion of their temperature coefficient, the bridge becomes unbalanced and the bridge output voltage is a measure of the air flow. One particular known diffused resistor anemometer provides a direct bridge output signal without amplification of two microvolts per millisecond of air flow across the bridge.
However, a significant disadvantage and limitation of a diffused resistor anemometer is that the four diffused resistors, using known integrated circuit processing techniques, cannot be made perfectly equal, and therefore the bridge will inherently be unbalanced. Thus, the Wheatstone bridge will also develop a signal as a function of changes in ambient temperature although there is no flow of the medium present across the integrated resistors.
Another type of integrated circuit anemometer utilizes the temperature sensitivity of P-N junction diodes, as disclosed in U.S. Pat. No. 3,992,940. Disclosed therein is a fluid flow sensor for measuring the mass flow of a fluid in a passageway. The sensor includes three solid state diodes wherein two are fabricated on a chip separate from the chip on which the third diode is fabricated. The first diode is responsive to force convective heat transfer by the fluid flowing thereover for generating a first electrical signal. The third diode is responsive to temperature of the fluid thereover for generating a second electrical signal. The second diode is responsive to both the first and second electrical signals for maintaining the first diode at a predetermined temperature above the third diode. The amount of current required to maintain the first diode at a predetermined temperature above the temperature of the third diode is proportional to the mass rate of the fluid flowing over the sensor.
A significant disadvantage and limitation of the diode type integrated anemometer is that two separate chips must be fabricated for each device. Another disadvantage and limitation of the diode type anemometer is that current required to maintain the diode a preselected temperature is not linearly dependent on fluid flow velocity, but the relationship has to be determined empirically. Each diode type anemometer will require different calibrations because of differences of devices from processing integrated circuits.
Another type of known anemometer utilizes the temperature sensitivity of the base-collector junction of a bipolar transistor. The measuring transistors are arranged in a differential amplifier configuration, with a heating transistor located intermediate the two sensing transistors. The differential output voltage measured across the collector of each of the measuring transistors will be a function of the temperature difference between the two measuring transistors. Typically, the transistors are disposed with one at the leading edge and the other at the trailing edge of the sensor with respect to the flow vector. Obviously, the transistor at the leading edge will be subject to greater cooling by the flow than the transistor at the trailing edge.
A significant disadvantage and limitation of the bipolar type sensor is that the temperature difference is proportional to the square root of the flow velocity at very low velocity, such as velocities less than one meter per second. The square root proportionality does not hold for larger flow velocities. Thus, the bipolar anemometer is not linear over the range of velocity which may be sensed. Also, a further disadvantage and limitation of the bipolar anemometer is that the unamplified differential voltage is too small to be useful for measuring. Subsequent amplification of the "raw" differential voltage which also amplifies calibration errors and noise creates inaccuracies in measuring fluid flow velocity.